1. Field of the Invention
The present invention relates to a coating method for applying a coating liquid, such as a liquid resist, to a substrate such as a semiconductor wafer.
2. Description of the Related Art
In a photolithography process in the manufacturing of a semiconductor device, process steps, including a resist coating step of applying a liquid resist e.g. onto a semiconductor wafer (hereinafter simply referred to as “wafer”) to form a resist film, an exposure step of exposing the resist film in a predetermined pattern and a developing step of developing the exposed resist film, are carried out sequentially to form a predetermined resist pattern on the wafer. In the resist coating step is employed a so-called spin coating method which involves supplying the liquid resist to approximately the center of the surface of the rotating wafer so as to spread the liquid resist on the wafer by centrifugal force, thereby coating the wafer surface with the liquid resist.
In such spin coating method, a wafer is held in a fixed state e.g. by vacuum attraction by means of a spin chuck, and the wafer is rotated together with the spin chuck by a rotary drive means while a liquid resist is dropped from a resist nozzle, disposed above the wafer, onto the center of rotation in the surface of the wafer. The liquid resist spreads radially outward on the wafer by centrifugal force. The wafer continues to be rotated after the dropping of the liquid resist is stopped so that the spread liquid resist is forced out of the wafer and the liquid resist on the wafer is dried.
Such a spin coating method is described, for example, in Japanese Patent Laid-Open Publication No. 2001-307984.
The above-described coating method, however, entails the following problems in carrying out resist coating of a substrate, such as a semiconductor wafer, by supplying a small amount of a liquid resist onto the substrate:
To meet the demand for finer and thinner semiconductor device patterns, various liquid resists which are adapted for such lithography have been developed. Because of the requirement of provision of a variety of physical properties in a liquid resist, the cost of liquid resists is rising faster than ever and is now very precious. The present situation therefore requires further reduction in the consumption of a liquid resist.
However, when a liquid resist is supplied in a small amount onto a substrate and the liquid resist spreads on the substrate outwardly from the center, the liquid resist is unlikely to spread uniformly in a peripheral region of the substrate surface and can spread, for example, like a beard. Once the liquid resist spreads like a beard and a resist film is formed in that pattern, it is not possible to subsequently make the thickness of the resist film uniform at every point. Thus, it has not been possible to uniformly apply a small amount of a liquid resist onto a wafer. Satisfactory reduction in the consumption of a liquid resist has therefore not been achieved as yet.